What it is: Basically a cord wrapped in bias tape placed in the seam between two of your fabric pieces. You can make your own by combining those two elements, or by pre-made bias tape.
Uses: Piping is seen most commonly on things like slipcovers, bags, and other non-wearable fashions. However, many old-fashioned patterns as well as some fresher looks use piping around the collar or to add decorative details.
Inserting piping in your project:
Check out the tutorial on Burda Style.